Conventional cathode ray tube displays are rapidly being replaced by liquid crystal displays (LCD) due to their mobility and compact size. Furthermore, as a next-generation display, organic electroluminescence (EL) displays that are self-luminous and are excellent in terms of luminosity, brightness, and power consumption are now being produced. Although these displays are excellent compared to the conventional cathode ray tube displays in terms of their mobility and compact size, they are relatively heavy and also break easily, which are their disadvantages, because glass is used as their substrate.
In order to solve these problems, a film substrate (which is called “placell”) is used in some liquid crystal displays. However, for those driven by a thin film transistor (TFT) that are currently mainstream and are capable of dealing with moving images, there are no film substrates available for practical use.
The reason for this is because there is no film substrate, which can resist the temperature for forming a TFT circuit and is also highly transparent. In the case of organic EL displays which are receiving much attention as a next-generation displays, considerably high oxygen barrier performance is also required in addition to the abovementioned characteristics in order to protect organic EL devices.
Clay thin films are attracting attention as a material that may satisfy these requirements. Clay thin films have excellent flexibility, and since they have a structure where clay particles are densely oriented in layers, they are materials with excellent gas barrier properties along with excellent heat resistance and nonflamability (refer to Patent Document 1). However, there are several problems associated with them when used as a film substrate for liquid crystal and organic EL displays.
One of the problems is surface flatness. By taking organic EL displays as an example, when the surface of a substrate is not flat, a transparent conductive film cannot be formed uniformly causing reduction in electrical conductivity at the defective parts. Furthermore, the organic EL device may also be damaged. Since the transparent conducting oxide and organic EL devices are formed of thin-film layers that are extremely thin and range from a few tens to a few hundreds of nanometers, their substrates are required to have low surface roughness of a few to a few tens of nanometers in order to prevent this problem. However, conventional clay thin films cannot satisfy this requirement. The reason for this lies in the production method thereof. Conventionally, clay thin films are prepared by allowing a clay dispersion, which is dispersed in a dispersion medium having water as a main component, to stand horizontally in order to slowly deposit clay particles as well as to evaporate the dispersion medium. In such a case, it is currently difficult to suppress values of surface roughness Ra down to a submicron range or less since the film surface is formed naturally.
Another problem of the conventional clay thin films is a problem of their water vapor barrier properties. Since clay is a hydrophilic material, it is hygroscopic and has a property to easily permeate water vapor. When clay thin films contain a large amount of water vapor, the densely layered and oriented structure of clay particles in the clay thin films is destroyed, and thus gas barrier properties of the clay thin films are impaired while their heat resistance and mechanical strength are also reduced. Since this problem originates from the nature of clay itself, it is difficult to improve.
[Patent Document 1]
Japanese Laid-Open Patent Application No. 2005-104133